1. Field of the Invention
The present invention relates to a liquid crystal display device having excellent viewing angle characteristics.
2. Related Art
Liquid crystal display devices are widely used in various applications of personal computer and mobile monitors and televisions, as having various advantages that they may be driven at low voltage and are power-saving and they may be small-sized and their bodies may be thinned. Various modes have been proposed for such liquid crystal display devices, depending on the alignment state of the liquid crystal molecules in the liquid crystal cell therein. Heretofore, a TN mode has been the mainstream for the devices, in which the crystal molecules are aligned as twisted by about 90° from the lower substrate toward the upper substrate of the liquid crystal cell.
In general, a liquid crystal display device comprises a liquid crystal cell, an optically-compensatory sheet and a polarizing element. The optically-compensatory sheet is for canceling image coloration and for enlarging viewing angle, for which used are a stretched birefringent film and a transparent film coated with liquid crystal. For example, Patent Reference 1 discloses a technique of applying an optically-compensatory sheet, which is prepared by applying a discotic liquid crystal onto a triacetyl cellulose film followed by aligning and fixing it, to a TN-mode liquid crystal cell thereby enlarging the viewing angle of the device. However, in liquid crystal display devices for application to televisions that are expected to watch at various angles on a large-size screen, the requirement for viewing angle dependency is serious; and even by the above-mentioned method, the serious requirement for them could not still be satisfied. Accordingly, liquid crystal display devices of other modes than the TN mode, such as an IPS (in-plane switching) mode, an OCB (optically-compensatory bent) mode or a VA (vertically-aligned) mode have been studied. In particular, a VA mode is specifically noted for liquid crystal display devices for TVs, as its contrast is high and its producibility is relatively high.
However, the VA mode enables a nearly completely black display in the normal line direction of panel, but has a problem in that, when the panel is observed in the oblique direction thereof, then it causes light leakage and the viewing angle is thereby narrowed. To solve the problem, proposed is use of an optically-biaxial retardation plate of which the refractive index differs all in the three-dimensional direction of the film, thereby to improve the viewing angle characteristics of the VA-mode device (for example, Japanese Laid-Open Patent Publication No. 2003-344856).
However, the light leakage is reduced only within a specific wavelength range (for example, green light at about 550 nm), and nothing is taken into consideration for the light leakage in any other wavelength range (for example, blue light at about 450 nm, red light at about 650 nm). Accordingly, for example, when the panel is observed obliquely in a black state, then the problem of color shift in blue or red coloration is not as yet solved.